Metal matrix composites are materials which comprise a secondary, reinforcing or filler phase in combination with a metal matrix. Metal matrix composites have different, and often improved or more desirable, properties as compared to their monolithic metal counterparts. For example, depending on the particular metal and reinforcing phases present in a composite material, as well as their respective ratios in the composite material, the composite material may have improved characteristics with respect to strength, stiffness, contact wear resistance and elevated temperature strength, as compared to the corresponding monolithic metal. Furthermore, depending on the choice of reinforcing phase present in the composite, the metal matrix materials may be less expensive to prepare than their monolithic metal counterparts.
There are many potential applications for metal matrix composites. Significant applications of metal matrix composites are likely in automotive components, machine parts, and electronic packaging, as well as in specialized products based on unique combinations of properties.
Of particular interest are metal matrix composites comprising fly ash, because such composites are less expensive to prepare and exhibit improved properties with respect to their corresponding monolithic metal counterparts. Fly ash is an abundant by-product that results from the combustion of pulverized coal. In the past fly ash has been employed as a concrete admixture, as a soil stabilizer, as a filler for asphalt and structural materials, such as bricks. Fly ash comprises micron sized, translucent spherical particles which consist primarily of alumina, silica, iron oxides, lime and manganese.
A variety of methods for producing metal matrix composite materials have been developed. These methods include diffusion bonding, powder metallurgy, casting, pressure infiltration of loose fly ash beds, spray codisposition and the like. For fly ash metal matrix composites in particular, stir casting and pressure infiltration of loose fly ash beds have found use.
Although a variety of methods for metal matrix composite material production have been developed, these methods are not entirely satisfactory. For example, problems with uneven distribution of the reinforcing phase in the metal matrix, inadequate levels of the reinforcing phase in the matrix, and lack of control over the amount of the reinforcing phase which is incorporated into the matrix, are known to occur. These problems are particularly prevalent in the production of fly ash metal matrix composites, where compositions with homogeneously distributed fly ash levels of 30% or higher have not heretofore been produced.
Thus, there is continued interest in the development of new methods of producing metal matrix composites. Of particular interest is the development of metal matrix composites comprising fly ash, wherein the fly ash levels exceed 30% of the composite and the fly ash is evenly distributed throughout the composite.
Relevant Literature
U.S. Pat. No. 5,226,494 reports a method of preparing a metal matrix composite material where the reinforcing phases, such as fly ash and oil ash, are added to molten metal and stirred at high speed. U.S. Pat. No. 5,020,584 reports a method of making metal matrix composite materials where molten metal infiltrates a preform of filler material, where an infiltration enhancer material is employed. Other U.S. Patents of interest include U.S. Pat. Nos. 3,573,940; 3,585,155; 4,601,832; 4,888, 054 and 4,936,270.